In many applications where polymers are used, it is necessary that the polymer composition and article fabricated from the polymer composition possess some flame retardance in order to minimize the danger of fire. For example, in electrical appliances, an electrical short can start a fire. The plastic housing, therefore, should be made of a material which does not burn easily.
For most polymer compositions, it is necessary to add materials to the composition that impart flame retardance to the article. Examples of materials which have been used to impart such flame retardance are brominated resins, antimony oxide fillers, and organic phosphates. These materials add significant costs to a polymer composition, and can have a negative impact on other properties, so it is desirable to limit flame retardants to the minimum amount necessary to impart flame retardance to the polymer composition.
Underwriters Laboratories has developed a variety of tests for materials which are to be used with or near electricity, including a flame test. The UL rating is very influential, and has become an industry standard. A polymer composition which does not have a UL flame rating will not be purchased for applications which require flame retardance. Likewise, a polymer composition which loses its UL flame rating will no longer be sold for the applications for which it was designed, those requiring flame retardance. Therefore, it is important that a new flame-retardant polymer composition receive a rating of flame retardant initially, and it is critical that the rating is maintained thereafter.
The Underwriters Laboratories flame test is designated UL-94. Briefly, one of the test protocols calls for exposing five bars of a polymer composition to a flame to initiate burning, removing the flame and recording the time for the flame to go out. The polymer composition tested can receive a rating of V-0 (most flame retardant), V-1, or V-2 (least flame retardant), depending on the results of the test.
Unfortunately, for a test that is vital to the commercial success of many polymer compositions, the UL-94 flame test yields results which can be notoriously variable. That is, there is high statistical variability associated with the results of the test. The variability has been attributed to differences in sample preparation, operator-to-operator variation, and even to changes in the burn mechanism.
Use of results from the UL-94 test to determine the amount of flame retardant to be added can lead to the addition of excessive amounts in order to guarantee compliance with the requirements of the test. Therefore, there is a need for an improved method for the determination of flame retardance of polymer compositions. In particular, there is a need for less variable, more quantifiable method of determining the performance of a polymer composition in the UL-94 test without changing the UL-94 test protocol.